Rivet_i Fixes, master branch (2020.03.20.)

Cleaned up the Rivet_i package a bit. This is to fix AGENE-1833.

I did the following:

• Changed the FastJetContrib dependency to just set up fjcontrib for the run- and link-time environment, but not to use ${FASTJETCONTRIB_INCLUDE_DIRS} and ${FASTJETCONTRIB_LIBRARIES} explicitly during the build of the package;
• Removed the build of the unnecessary shared library;
  • Moved all the sources under src/ at the same time, since the package doesn't provide any public headers;
• Added the missing RivetEnvironmentConfig.cmake file;
  • Making it a bit prettier compared to the version sitting in 21.6 at the moment.

This is meant to do the same as !31357 (closed), but I believe in a bit nicer way. 😛 (I only saw !31357 (closed) once I was ready with these changes, so I thought I should create an MR nevertheless...)